High temperature fuel cell system and method of operating same

ABSTRACT

A high temperature fuel cell stack system, such as a solid oxide fuel cell system, with an improved balance of plant efficiency includes a thermally integrated reformer, combustor and the fuel cell stack.

This application is a divisional application of U.S. application Ser.No. 12/222,712, filed Aug. 14, 2008, which is a divisional of U.S.application Ser. No. 11/002,681, filed Dec. 3, 2004, which claims thebenefit of priority of U.S. provisional applications 60/537,899 filed onJan. 22, 2004 and 60/552,202 filed on Mar. 12, 2004, all of which areincorporated herein by reference in their entirety.

BACKGROUND OF THE INVENTION

The present invention is generally directed to fuel cells and morespecifically to high temperature fuel cell systems and their operation.

Fuel cells are electrochemical devices which can convert energy storedin fuels to electrical energy with high efficiencies. High temperaturefuel cells include solid oxide and molten carbonate fuel cells. Thesefuel cells may operate using hydrogen and/or hydrocarbon fuels. Thereare classes of fuel cells, such as the solid oxide regenerative fuelcells, that also allow reversed operation, such that oxidized fuel canbe reduced back to unoxidized fuel using electrical energy as an input.

BRIEF SUMMARY OF THE INVENTION

The preferred aspects of present invention provide a high temperaturefuel system, such as a solid oxide fuel cell system, with an improvedbalance of plant efficiency. The system includes a thermally integratedunit including a reformer, combustor and the fuel cell stack.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1, 2 and 7 are schematics of fuel cell systems according topreferred embodiments of the present invention.

FIGS. 3 and 6 are schematics of PSA gas separation devices according tothe preferred embodiments of the present invention.

FIG. 4 is a schematic of a fuel cell system according to the fifthpreferred embodiment of the present invention.

FIG. 5 shows the details of the system of FIG. 4.

FIGS. 8A and 8B are schematics of integrated cylindrical reformer,combustor and stack unit for a system with two stacks.

FIGS. 9A and 9B are schematics of integrated plate type reformer,combustor and stack unit for a system with two stacks.

FIGS. 10A and 10B are schematics of integrated plate type reformer,combustor and stack unit for a single stack system.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 illustrates a fuel cell system 1 according to the preferredembodiments of the present invention. Preferably, the system 1 is a hightemperature fuel cell stack system, such as a solid oxide fuel cell(SOFC) system or a molten carbonate fuel cell system. The system 1 maybe a regenerative system, such as a solid oxide regenerative fuel cell(SORFC) system which operates in both fuel cell (i.e., discharge) andelectrolysis (i.e., charge) modes or it may be a non-regenerative systemwhich only operates in the fuel cell mode.

The system 1 contains a high temperature fuel cell stack 3. The stackmay contain a plurality of SOFCs, SORFCs or molten carbonate fuel cells.Each fuel cell contains an electrolyte, an anode electrode on one sideof the electrolyte in an anode chamber, a cathode electrode on the otherside of the electrolyte in a cathode chamber, as well as othercomponents, such as separator plates/electrical contacts, fuel cellhousing and insulation. In a SOFC operating in the fuel cell mode, theoxidizer, such as air or oxygen gas, enters the cathode chamber, whilethe fuel, such as hydrogen or hydrocarbon fuel, enters the anodechamber. Any suitable fuel cell designs and component materials may beused.

The system 1 contains any one or more of the following elements, eitheralone or in any suitable combination. In a first embodiment, the system1 contains a desulfurizer 5 and a water-gas shift reactor 7 that arethermally integrated with each other. The waste heat from the reactor 7is used to heat the desulfurizer 5 to its operating temperature, thusreducing or eliminating the need for a separate desulfurizer heater.

In a second embodiment, the system 1 contains a hydrocarbon fuelreformer 9 and a hydrocarbon fuel by-pass line 11 fluidly connected tothe fuel inlet 13 of the high temperature fuel cell stack 3. The by-passline 11 by-passes the reformer 9 to provide unreformed hydrocarbon fuelinto the fuel inlet 13 of the high temperature fuel cell stack 3 tocontrol the temperature of the stack 3.

In a third embodiment, the hydrocarbon fuel reformer 9 is locatedseparately from but thermally integrated with the high temperature fuelcell stack 3. A start-up burner 15 is thermally integrated with thereformer 9. Preferably but not necessarily, a start-up burner 15effluent outlet conduit 17 is fluidly connected to an oxidizer inlet 19of the high temperature fuel cell stack 3. This configuration allows thesystem 1 to be started up using only the hydrocarbon fuel and oxidizerwithout oxidizing SOFC anode electrodes. This configuration eliminates aseparately stored reducing or inert purge gas that is flushed throughthe system to prevent the anode electrodes of the SOFCs from oxidizing.

In a fourth embodiment, a carbon monoxide separation device 21, such aspressure swing adsorption (PSA) device is fluidly connected to a fuelexhaust (i.e., the fuel outlet) 23 of the stack 3. A carbon monoxiderecycle conduit 25 has an inlet that is connected to the outlet of thecarbon monoxide separation device 21 and an outlet that is fluidlyconnected to a fuel inlet 13 of the high temperature fuel cell stack 3.For example, the device 21 allows carbon monoxide to be separatelyrecirculated into a hydrocarbon fuel inlet conduit 27 of the stack 3 toenhance the electrochemical reaction in the fuel cells of the stack 3.Furthermore, since carbon monoxide is recirculated, less carbon monoxideis provided into the atmosphere than if the carbon monoxide from thesystem was simply flared or vented into the atmosphere.

In a fifth preferred embodiment, a PSA hydrogen separation device 29 isfluidly connected to the fuel exhaust 23 of the stack 3. A thermaloutput of the high temperature fuel stack 3 in addition to the fuelexhaust is thermally integrated with at least a first column of the PSAdevice 29 to heat the first column. This use of the stack 3 waste heatto heat a PSA column under purge allows a reduction in the compressionrequirements of the PSA device and/or an increase in the amount of gaspurification for the same level of compression.

In a sixth preferred embodiment, a solid oxide fuel cell system with animproved balance of plant efficiency comprises a thermally integratedreformer, combustor and stack, where the reformer is heated by the stackcathode exhaust, by radiative and convective heat from the stack and bythe combustor heat during steady state operation. In a seventh preferredembodiment, the system starts up with hydrogen generated using a CPDX(catalytic partial oxidation) reactor. In an eighth preferredembodiment, the system contains an energy efficient and self sufficientwater management subsystem. The system contains at least one evaporatorwhich uses stack anode exhaust to heat water being provided into theinlet fuel stream.

I. First Embodiment

The elements of the system 1 of the first embodiment will now bedescribed. In prior art systems, organo-sulfur compounds (e.g.,mercaptans, thiophenes) contained in natural gas fuel are hydrogenatedby adding hydrogen to the fuel inlet stream and reacting the mixture ina desulfurizer over a suitable catalyst, such as cobalt-molybdenum. Thereaction produces CH₄ and H₂S gases. The H₂S gas is subsequently removedby reaction with a fixed sorbent bed, containing for example ZnO orother suitable materials for removing this gas. Usually these reactionsare carried out at about 300° C., and the catalyst and sorbent can becontained in the same vessel.

FIG. 1 illustrates an embodiment of a desulfurizer subsystem of thefirst embodiment, which comprises the desulfurizer 5 and the water-gasshift reactor 7 that are thermally integrated with each other.

The desulfurizer 5 preferably comprises the catalyst, such as Co—Mo orother suitable catalysts, which produces CH₄ and H₂S gases fromhydrogenated, sulfur containing natural gas fuel, and a sorbent bed,such as ZnO or other suitable materials, for removing the H₂S gas fromthe fuel inlet stream. Thus, a sulfur free or reduced sulfur hydrocarbonfuel, such as methane or natural gas leaves the desulfurizer 5.

The water-gas shift reactor 7 may be any suitable device which convertsat least a portion of the water exiting the fuel cell stack 3 fuelexhaust 23 into free hydrogen. For example, the reactor 7 may comprise atube or conduit containing a catalyst which converts some or all of thecarbon monoxide and water vapor in the tail gas exiting exhaust 23through a fuel exhaust conduit 31 into carbon dioxide and hydrogen. Thecatalyst may be any suitable catalyst, such as an iron oxide or achromium promoted iron oxide catalyst. The reactor 7 is preferablylocated along conduit 31 between the fuel exhaust 23 and the PSAhydrogen separation device 29. The reactor 7 works in tandem with thePSA hydrogen separation device 29 by increasing the amount of freehydrogen in the fuel side exhaust (i.e., anode exhaust or tail gas) byconverting some water present in the fuel side exhaust gas intohydrogen. The reactor 7 then provides hydrogen and carbon dioxide to thePSA hydrogen separation device 29, which separates the hydrogen from thecarbon dioxide. Thus, some of the water present in the fuel may beconverted to hydrogen in the reactor 7.

The desulfurizer 5 and the water-gas shift reactor 7 are thermallyintegrated with each other. This means that waste heat from the reactor7 is transferred directly or indirectly to the desulfurizer 5. Forexample, the desulfurizer 5 and the water-gas shift reactor 7 may belocated in the same hot box such that they are thermally integrated witheach other. Alternatively, the desulfurizer 5 and the water-gas shiftreactor 7 may be located in thermal contact with each other (i.e., indirect physical contact or by contacting the same thermal mass).Alternatively, the desulfurizer 5 and the water-gas shift reactor 7 maybe connected by a thermal conduit, such as a pipe containing a thermaltransfer fluid, such as water/steam or another fluid.

The desulfurizer 5 is fluidly connected to the fuel inlet 13 of the fuelcell stack 3. The water-gas shift reactor 7 is fluidly connected to thefuel exhaust 23 of the fuel cell stack. The term fluidly connected meansthat the connection may be direct or indirect, as long as a gas orliquid fluid may be provided through the connection. Preferably, thedesulfurizer 5 is connected to the fuel inlet 13 of the fuel cell stack3 by the fuel inlet conduit 27. A valve 28 controls the flow of fuelthrough conduit 27. The water-gas shift reactor 7 is connected to thefuel exhaust 23 of the fuel cell stack 3 by the fuel exhaust conduit 31.It should be noted that the conduits 27 and 31 shown in FIG. 1 containseveral portions or sections that are separated by various processingdevices, such as the desulfurizer 5 and the water-gas shift reactor 7.

A method of operating the fuel cell system 1 according to the firstembodiment includes providing a hydrocarbon fuel, such as natural gas,into a desulfurizer 5 from a hydrocarbon fuel source 33, such as anatural gas supply pipe or a hydrocarbon fuel storage tank or vessel.The fuel is desulfurized in the desulfurizer 5 and is then provideddirectly or indirectly into the fuel cell stack 3 through conduit 27 andinlet 13, as will be described in more details with respect to the otherembodiments of the present invention.

The warm fuel exhaust is provided from the fuel cell stack 3 throughexhaust 23 and conduit 31 into the water-gas shift reactor 7.Preferably, the exhaust has given up some of the heat in heat exchangersand other devices prior to entering the reactor. For example, anoptional fuel heat exchanger and water vaporizer 35 may be providedbetween the conduits 27 and 31. The vaporizer 35 humidifies the fuelinlet stream in conduit 27. The vaporizer 35 can be a device whichsupplies water vapor based on cyclic desiccant beds or a rotatingdesiccant wheel (i.e., “enthalpy wheel”) and which provides water vaporfrom an exhaust stream in conduit 31 into the fuel inlet stream inconduit 27 or the vaporizer 35 can be a steam generator which provideswater vapor into the fuel inlet stream from another water source.

The warm exhaust gases react with each other according to the forwardwater gas shift reaction, CO+H₂O—>CO₂+H₂, in the reactor 7 and give upor provide heat to the desulfurizer 5 side and the incoming fuel gasespassing through it. Preferably, the reactor 7 supplies all of the heatthat is needed to operate the desulfurizer 5 at its standard operatingtemperature, such as at least about 300° C., and no other heating meansor heaters are used to heat the desulfurizer 5.

The hydrogen, carbon monoxide, carbon dioxide and water vapor containingexhaust continues in conduit 31 to an optional condenser 37, optionalwater knockout or separation device 39 and a compressor 41, such as areciprocating compressor, to the PSA hydrogen 29 and/or carbon monoxide21 separation devices. The water knock out system 39 separates the waterfrom the fuel exhaust stream and discharges it out of the waterdischarge conduit 43 controlled by valve 45, or recirculates it into thefuel heat exchanger and water vaporizer 35 using a positive displacementpump 47. The water is preferably provided into the stack 3 when thestack 3 is operated to generate hydrogen while generating little or noelectricity in the fuel cell mode (i.e., no net electricity is producedin the fuel cell mode), as will be described in more detail below. Theadditional water is used to support fuel reforming as needed. Anoptional water inlet conduit 49 may also be connected to the waterknockout device 39. Additional water may be provided for bootstrapping,as the electrochemical process will ordinarily generate net water. Aflow control valve 51 position is controlled mechanically or by acomputer dependent on the water level in the water knockout device. Thevalve 51 controls the amount of water provided into device 39 throughconduit 49 based on the water level in the device 39.

II. Second Embodiment

FIG. 1 illustrates a system 1 of the second embodiment containing thehydrocarbon fuel by-pass line 11 which allows feeding the fuel cellstack 3 with an accurately controlled fuel input mixture for improvedcontrol of broad range of operating conditions, such as stack 3operating temperature.

In the prior art, SOFCs are commonly operated with hydrocarbon fuels,such as methane. The methane may be partially or fully steam reformed toform hydrogen and carbon oxides before it enters the SOFC stack. Steamreformation is an endothermic process. If methane is only partiallysteam reformed, the remaining methane will be reformed within the SOFC.The endothermic reaction within the SOFC stack affects the thermalbalance of the SOFC stack.

Oxidation of hydrocarbons in the high temperature fuel cells includes anendothermic reaction in which the hydrocarbon fuel, such as methane ornatural gas, is converted to hydrogen and carbon oxides. Thisendothermic reaction may not be obvious in the net reaction occurring inthe fuel cell system. One example is a solid oxide fuel cell fed withmethane. In the net reaction, the methane is oxidized to carbon dioxideand water. However, it is electrochemically highly unlikely for themethane to be directly oxidized to the products. It is commonly assumedthat some methane first reacts with steam, which is almost alwayspresent, to form hydrogen and carbon oxides. Then the hydrogen and lowercarbon oxides are oxidized. The water formed by the oxidation ofhydrogen can enable steam reformation of yet unconverted methane. Theresult of this intermediate chemical reaction is heat consumption at thelocation of the steam reformation.

Heat consumed by reformation inside the fuel cell thereby creates acooling effect. This cooling effect can be localized and createtemperature gradients and in turn thermal stresses which can damage partof the fuel cell. Thus, the reformation is partly or completelyperformed outside the fuel cell stack.

In the system 1 of the second embodiment, the hydrocarbon fuel reformer9 is located separately from the high temperature fuel cell stack 3. Thereformer is adapted to at least partially reform a hydrocarbon fuel intoa hydrogen fuel. The hydrocarbon fuel is provided into the reformer 9through the hydrocarbon fuel inlet conduit 27, connected to an inlet ofthe reformer 9. A connecting conduit 53 connects the fuel inlet 13 ofthe high temperature fuel cell stack 3 with an outlet of the reformer 9.

The hydrocarbon fuel reformer 9 may be any suitable device which iscapable of partially or wholly reforming a hydrocarbon fuel to form acarbon containing and free hydrogen containing fuel. For example, thefuel reformer 9 may be any suitable device which can reform ahydrocarbon gas into a gas mixture of free hydrogen and a carboncontaining gas. For example, the fuel reformer 9 may reform a humidifiedbiogas, such as natural gas, to form free hydrogen, carbon monoxide,carbon dioxide, water vapor and optionally a residual amount ofunreformed biogas. The free hydrogen and carbon monoxide are thenprovided into the fuel inlet 13 of the fuel cell stack 3 through conduit53.

In a preferred aspect of the second embodiment, the fuel reformer 9 isthermally integrated with the fuel cell stack 3 to support theendothermic reaction in the reformer 9 and to cool the stack 3. The term“thermally integrated” in this context means that the heat from thereaction in the fuel cell stack 3 drives the net endothermic fuelreformation in the fuel reformer 9. The fuel reformer 9 may be thermallyintegrated with the fuel cell stack 3 by placing the reformer 9 andstack 3 in the same hot box and/or in thermal contact with each other,or by providing a thermal conduit or thermally conductive material whichconnects the stack 3 to the reformer 9. While less preferred, a separateheater may also be used to heat the reformer 9 instead of or in additionto the heat provided from the stack 3.

The hydrocarbon fuel by-pass line 11 is fluidly connected to the fuelinlet 13 of the high temperature fuel cell stack 3. In other words, theby-pass line may be connected directly to the inlet 13 or it may beindirectly connected to the inlet 13 via the connecting conduit 53. Theterms “line” and “conduit” are used interchangeably, and include gasflow pipes and other fluid flow ducts. The by-pass line 11 is adapted toprovide unreformed hydrocarbon fuel into the fuel inlet 13 of the hightemperature fuel cell stack 3.

Preferably, the by-pass line 11 branches off from the hydrocarbon fuelinlet conduit 27 upstream of the reformer 9 and connects to theconnecting conduit 53 downstream of the reformer 9. Preferably, thedesulfurizer 5 is located upstream of a location where the by-pass line11 branches off from the hydrocarbon fuel inlet conduit 27, such thatsulfur is removed from the unreformed hydrocarbon fuel provided throughthe by-pass line 11.

Alternatively, the by-pass line 11 does not have to branch off from thefuel inlet conduit 27. In this case, the by-pass line 11 is connected toa separate source of hydrocarbon fuel, such as a natural gas pipe or astorage vessel. In this case, a separate desulfurizer is provided in theby-pass line 11.

The system 1 further comprises a hydrocarbon fuel flow control valve 55in the by-pass line 11. The control valve 55 is adapted to control aflow of the unreformed hydrocarbon fuel into the fuel cell stack tocontrol a temperature and/or other operating parameters of the fuel cellstack. The valve 55 may be manually or remotely controlled by anoperator. Alternatively, the valve 55 may be automatically controlled bya computer or other processing device. The valve 55 may be controlledautomatically or by an operator in response to detected or predeterminedparameters. For example, the temperature or other parameters of the fuelcell stack 3 may be detected by a temperature detector or otherdetectors, and the results provided to an operator or a computer. Theoperator or computer then adjust the valve 55 to control the flow of theunreformed hydrocarbon fuel through the by-pass line 11 into the fuelcell stack 3 to control the fuel cell stack temperature or otherparameters. Alternatively, the by-pass valve 55 may be adjusted based onpredetermined parameters, such as based on time of stack 3 operationthat is stored in the computer memory.

A method of operating a high temperature fuel cell system 1 of thesecond embodiment includes providing a hydrocarbon fuel into a reformer9 through the fuel inlet conduit 27 and at least partially reforming thehydrocarbon fuel into hydrogen fuel in the reformer 9. The hydrogen fuelfrom the reformer 9 is provided into a fuel inlet of a high temperaturefuel cell stack 3 through conduit 53. The unreformed hydrocarbon fuelthat does not pass through the reformer 9 is provided into the fuelinlet of the high temperature fuel cell stack 3 through the by-pass line11 and optionally through conduit 53. The flow of the unreformedhydrocarbon fuel through the by-pass line 11 that does not pass throughthe reformer 9 is controlled by the valve 55 to control a temperatureand/or other operating parameters of the high temperature fuel cellstack 3.

The oxidation of hydrogen or low carbon oxides inside a fuel cell is anexothermic reaction. Heat generated by the exothermic oxidation shouldbe removed to attain stable operation. Unreformed methane can aid in theremoval of heat via the above described steam reformation in thereformer 9.

Accurately controlling the amount of unreformed hydrocarbons enteringthe stack allows control of the temperature of the fuel cells in thestack. In an external reformer, the degree of reformation depends on avariety of factors some of which may vary during operation. Thus, asimple way of controlling the amount of unreformed hydrocarbons enteringthe stack is the by-pass line 11 which by-passes the external reformer 9as shown in FIG. 1. The bypass valve 55 controls the amount ofunreformed hydrocarbons entering the stack 3. More specifically, theleast amount of hydrocarbons entering the stack 3 is limited by thefinite conversion inside the external reformer. An upper limit for theamount of unreformed hydrocarbons entering the stack 3 is posed byreformation occurring outside the external reformer along by-pass line11.

This method of the second embodiment is applicable not only to solidoxide fuel cells, but any high temperature fuel cell fed by a fuel whichundergoes reformation reactions prior to oxidation. In the exampleprovided above, the reformation is an endothermic reaction, but thereare also reformation reactions that are exothermic. Examples ofexothermic reformation include but are not limited to partial oxidationof methane. Methane, which is the major constituent of natural gas, is avery common example of a hydrocarbon fuel that undergoes reformation,but other hydrocarbon fuels, such as natural gas, propane and butane arepossible. Therefore the reformer by-pass 11 is applicable to variousreformation reactions and a variety of hydrocarbon fuels.

FIG. 2 illustrates a system 101 of the first and second embodimentshaving an alternative configuration from the system 1 shown in FIG. 1.In the system 101 shown in FIG. 2, the fuel heat exchanger and watervaporizer 35 is located upstream of the desulfurizer 5. Thus, the fuelenters the heat exchanger and water vaporizer 35 prior to entering thedesulfurizer. The desulfurizer 5 and the water-gas shift reactor 7 arelocated in the same hot box 103 (i.e., a warm box or fuel conditioner).The fuel conditioner 103 also contains the catalytic start-up burner 15,the fuel heat exchanger and water vaporizer 35 and an optional fuelpre-reformer 105 which is thermally integrated with a fuel exhaust heatexchanger 107. Thus, the catalytic start-up burner 15 in the system 101is located in the fuel conditioner 103 rather than thermally integratedwith the fuel cell stack 3. The start-up burner 15 is fed by the stackfuel exhaust conduit 31 rather than by the hydrocarbon fuel inletconduit 27.

The fuel cell stack 3 is thermally integrated with the reformer 9 in thesame hot box 108. The hot box 108 also contains an optional radiativeair heater 109 and an optional catalytic tail gas burner 110. The tailgas burner 110 is provided with hydrocarbon fuel from a branch 111 ofthe hydrocarbon fuel inlet conduit 27, which is regulated by valve 112.A valve 113 directs the fuel exhaust flow between the PSA hydrogenseparation device 29 and the tail gas burner 110. The oxidizer sidecomponents are the same as in system 1 and will be described in moredetail below.

III. Third Embodiment

FIG. 1 illustrates a system 1 of the third embodiment, where the systemis brought up from room temperature to operating conditions (i.e., atstart-up) with only hydrocarbon fuel and air. No stored nitrogen orhydrogen are required to protect the anodes from oxidation. The stack 3,reformer 9 and start-up burner 15 comprise separate devices which arethermally integrated with each other, such as being located in the samehot box and/or being in thermal contact with each other and/or beingconnected by a thermal fluid transfer conduit.

The anodes of SOFC systems are commonly made from materials includingmetal oxides which have to be reduced to attain electron conductivityand thereby enable the electrochemical reaction in the anode chamber.Metallic oxides used include, but are not limited to nickel oxide. Onecommon difficulty with these metallic oxides is the necessity to keepthem reduced once they have been reduced. Re-oxidation causessignificant if not catastrophic performance degradation. The preventionof re-oxidation is a technical challenge for the start-up of a SOFCsystem.

Commonly, the fuel cell anode chamber is flushed with inert or reducinggases, such as nitrogen or hydrogen, at start-up to prevent anodeelectrode re-oxidation. For systems operated on hydrocarbon fuels,nitrogen or hydrogen are not readily available. Instead one or bothgases are stored separately within or near the system for consumptionduring start-up.

The inventor has realized that a SOFC system can be built and operatedsuch that the anode can be maintained in its reduced state while thesystem is operating only on hydrocarbon fuel and air during start-up.FIG. 1 illustrates one configuration of a SOFC system 1 according to thethird embodiment.

As shown in FIG. 1, the start-up burner 15 is thermally integrated withthe reformer 9 and the reformer 9 is thermally integrated with the stack3. As discussed above, the term “thermally integrated” includes locationin the same hot box, located in thermal contact with each other and/orconnection by a thermal transfer fluid conduit. The hydrocarbon fuelinlet conduit 27 is fluidly connected to an inlet of the reformer 9 andto a first inlet of the start-up burner 15 (such as via the burner fueldelivery conduit 73). A burner oxidizer inlet conduit 57 is connected toa second inlet of the start-up burner 15. The start-up burner 15effluent outlet conduit 17 is fluidly connected to an oxidizer inlet 19of the high temperature fuel cell stack 3. For example, the effluentoutlet conduit 17 may be connected directly into the oxidizer inlet 19of the stack or it may be connected to an oxidizer inlet conduit 59which is connected to the oxidizer inlet 19 of the stack 3. The systemalso contains an oxidizer blower 61, such as an air blower, whichprovides the oxidizer into the burner and stack through the conduits 57and 59, and an oxidizer exhaust conduit 63 which removes oxidizerexhaust from the stack 3. The system also contains an optional airfilter 65 and an optional heat exchanger 67 which heats the oxidizerbeing provided into the stack 3 through the oxidizer inlet conduit 59using the heat of the oxidizer exhaust being provided through theoxidizer exhaust conduit 63. The system also contains valves 69 and 71in conduits 57 and 59, respectively. The system also contains a burnerfuel delivery conduit 73 which is regulated by valve 75. The conduit 73branches off from conduit 27 or it may comprise a separate fuel deliveryconduit.

The method of operating the system 1 according to the third embodimentis as follows. Initially all components of the system are at roomtemperature. The anodes of the SOFC stack are reduced from previousoperation. Valves 69 and 75 in conduits 57 and 73, respectively, areopen, while valve 71 in conduit 59 is closed.

Hydrocarbon fuel, which can be natural gas, and an oxidizer, such as airor oxygen, are injected into the start-up burner 15 through conduits 73and 57, respectfully. The fuel and oxidizer are ignited in the burner 15by an ignitor. The heat stream from the combustion in the burner 15 isdirected at the thermally integrated reformer 9. The effluent of theburner 15 is directed through conduits 17 and 59 into the oxidizer inlet19 (i.e., cathode chamber(s)) of the stack 3. The effluent is exhaustedfrom the stack 3 through conduit 63 and from there into the air heatexchanger 67. It may be advantageous to operate the burner lean on fuelto ensure that the cathode chamber(s) of the fuel cells in the stack arealways exposed to an oxidizing environment. The combustion raisesprimarily the temperature of the reformer 9, secondarily the stack 3temperature, and finally the air heat exchanger 67 temperature.

Fuel is directed through the desulfurizer 5, vaporizer 35 and thereformer 9, into the stack 3 fuel inlet 13 (i.e., into the anodechamber(s) of the fuel cells in the stack). The fuel may be directedinto inlet 13 simultaneously with injecting fuel and oxidizer into theburner 15. Alternatively, the fuel may be directed into the inlet 13after it is directed into the burner 15 but before the stack 3 reachesthe anode oxidation temperature. The fuel exhaust exits the stack 3through conduit 31 and water-gas shift reactor 7. The fuel picks up heatin the reformer 9 and in the stack 3 and transports the heat to thewater gas shift reactor 7 which is thermally coupled to the desulfurizer5. Thereby the desulfurizer 5 is heated to operating conditions by thestack 3 exhaust, as provided in the first embodiment. Some heat is alsodelivered to the water vaporizer 7.

The heat balance in the system is designed such that the reformer 9reaches its operating temperature (i.e., the temperature at which itreforms the hydrocarbon fuel) before the stack 3 anode chamber(s)reaches a temperature were re-oxidation of the anode electrodes couldoccur. Also, enough heat is carried to the water vaporizer 35 such thatthe inflowing fuel is sufficiently humidified to avoid carbon formationin the hot components downstream of the water vaporizer 35.

If these method parameters are satisfied, the SOFC anode electrodes willbe exposed to a hydrogen rich feed stream created by steam reformationin the reformer 9 when a temperature at the anode is reached at whichre-oxidation can occur. At the same time, the desulfurizer 5 is broughtto operating temperature early enough to avoid detrimental effects ofexcessive sulfur content in the fuel feed stream. One preferredhydrocarbon fuel for this system is natural gas. However other fuels,including but not limited to methane, propane, butane, or even vaporizedliquid hydrocarbons can be utilized.

After the start-up of the system 1 is completed (i.e., once the stackreaches desired steady state operating conditions), hydrocarbon fuel andoxidizer supply into the start-up burner is terminated either by anoperator or automatically by a computer and the start-up burner isturned off. Valves 69 and 75 are closed and valve 71 is opened toprovide an oxidizer into the oxidizer inlet 19 of the solid oxide fuelcell stack 3. The stack 3 then operates in the fuel cell mode togenerate electricity from an electrochemical reaction of the fuelprovided through conduit 27 and reformer 9 (and optionally throughby-pass line 11) and oxidizer provided through conduit 59. Thus, itshould be noted that the oxidizer is preferably not provided intooxidizer inlet 19 of the solid oxide fuel cell stack 3 while thestart-up burner 15 operates. Furthermore, no separately stored reducingor inert gases are used to flush the anode chambers of the solid oxidefuel cells of the stack 3 during the start-up.

IV. Fourth Embodiment

FIG. 1 illustrates a system 1 of the fourth embodiment, where the carbonmonoxide and hydrogen are separately extracted from the fuel exhauststream and recirculated into the fuel inlet stream and/or removed fromthe system for other use.

The system 1 contains a hydrogen separation or purification device 29fluidly connected to a fuel exhaust 23 of the stack 3. Preferably, thedevice 29 comprises a PSA device that is connected to the exhaust 23 ofthe stack 3 by the fuel exhaust conduit 31. The PSA device 29 is adaptedto separate at least a portion of hydrogen from the fuel exhaust whilethe fuel cell stack 3 generates electricity in a fuel cell mode. Areciprocating pump 41 provides the fuel exhaust into the PSA device 29.The hydrogen separation or purification device 29 preferably containsthe carbon monoxide separation device or unit 21 and a carbondioxide/water separation device or unit 30. Preferably, the devices 21and 30 comprise PSA devices which respectively separate carbon monoxideand carbon dioxide/water from the fuel exhaust and allow hydrogen topass through. Preferably, but not necessarily, the PSA device 21 islocated in series with and downstream from PSA device 30. However, ifdesired, the PSA device 21 may be located upstream of PSA device 30.Preferably, PSA devices 21 and 30 comprise different units of a singlePSA system 29, where each unit 21 and 30 contains at least two PSAcolumns.

A carbon monoxide recycle conduit 25 recirculates the carbon monoxidefrom the PSA device 21 into the fuel inlet conduit 27. A carbondioxide/water removal conduit 26 removes carbon dioxide and water fromdevice 30. The inlet of conduit 25 is connected to the outlet of thecarbon monoxide separation device 21 and the outlet of conduit 25 isfluidly connected to the fuel inlet of the high temperature fuel cellstack 3, such as via the fuel inlet conduit 27. Alternatively, theoutlet of the carbon monoxide recycle conduit 25 may be connecteddirectly into the reformer 9 and/or into the fuel inlet 13 of the stack3.

The system further comprises an optional hydrogen recycle conduit 77which is controlled by a recirculated hydrogen flow control valve 79.The inlet of the conduit 77 is connected to the outlet of the hydrogenseparation device 29. The outlet of the conduit 77 is fluidly connectedto the fuel inlet 13 of the high temperature fuel cell stack 3, such asdirectly connected to the inlet 13 or indirectly connected via the fuelinlet conduit 27. If desired, the conduit 77 may be omitted and hydrogenand carbon monoxide may be carried together through conduit 25, whichmay result in a higher purity product or lower compression requirements.If desired, the flow of hydrogen passed through conduit 25 may bemetered.

Preferably, the outlet of conduits 25 and 77 are merged together into arecycle conduit 81, which provides the recirculated hydrogen and carbonmonoxide into the fuel inlet conduit 27, as shown in FIG. 1, or directlyinto the reformer 9 and/or the stack 3 fuel inlet 13. However, therecycle conduit 81 may be omitted, and the conduits 25 and 77 mayseparately provide carbon monoxide and hydrogen, respectively, intoconduit 27, reformer 9 and/or stack 3 fuel inlet 13. The system 1 alsooptionally contains a hydrogen removal conduit 83, which removeshydrogen from the system 1 for storage or for use in a hydrogen usingdevice, as will be described in more detail below.

FIG. 3 illustrates an exemplary two column PSA hydrogen separationdevice 29, such as, for example, the carbon dioxide/water separationunit 30 of a larger device or system 29 which also contains the carbonmonoxide separation unit 21 shown in FIG. 1. Preferably, the PSA device29 operates on a Skarstrom-like PSA cycle. The classic Skarstrom cycleconsists of four basic steps: pressurization, feed, blowdown, and purge.The device 29 contains two columns 83 and 85. When one column isundergoing pressurization and feed, the other column is undergoingblowdown and purge. In one exemplary configuration, the device 29 may beoperated using three three-way valves 87, 89 and 91 and one or more flowrestrictors 93. Of course other configurations may also be used. Whenthe three-way valves are in the positions shown in FIG. 3, thepressurization and feed steps are essentially combined, and the blowdownand purge steps are similarly combined.

The PSA device 29 operates as follows. A pressurized feed gas (F), suchas the fuel exhaust gas, containing CO₂, H₂O, CO and H₂ is providedthrough the fuel exhaust conduit 31. Two-position, three-way valves 87,89 and 91 are simultaneously switched to the state shown in FIG. 3A. Thefeed is introduced to column 83 via valve 87 pressurizing column 83. Theadsorbent contained in column 83 selectively adsorbs the CO₂ and H₂O. Asthe feed continues to flow, most of the H₂ exits as extract (E) viavalve 91. The extract may be provided into conduits 77 and/or 83 shownin FIG. 1, or into the carbon monoxide PSA device 21 to remove carbonmonoxide from the extract. The device 21 operates in the same way as theportion of the device 29 shown in FIG. 3, except the bed materials inthe columns are selected to separate hydrogen from carbon monoxide.

The switching of valve 89 exposes column 85 to a low pressure line,resulting in the blowdown of column 85. The low pressure line 95 is theoutput of column 83 that passes through one or more flow restrictors 93.Gases that were previously adsorbed during the previous cycle desorb andflow out through valve 89, producing a desorbate stream (D). Thedesorbate stream may be provided into the carbon monoxide PSA device 21to remove carbon monoxide from the desorbate stream. Meanwhile, arelatively small quantity of high pressure extract gas flows through theflow restrictor(s) 93 and through column 85 in the direction opposite tothe feed flow, forming a purge flow that helps remove desorbate fromcolumn 85.

At a subsequent time, as column 83 approaches saturation, the positionsof all valves are switched. Thus, column 85 becomes the column fed viavalve 87 and is pressurized, and column 83 vents via valve 89 and blowsdown. In this way the purity of the extract gas E is maintained.

A method of operating the system 1 of the fourth embodiment will now bedescribed. A fuel and an oxidizer are provided into the fuel cell stack3 through conduits 27/53 and 59, respectively. A fuel side exhauststream is generated from the fuel cell stack 3 through conduit 31 whilethe fuel and the oxidizer are provided into the fuel cell stack 3operating in a fuel cell mode (i.e., while the stack is generatingelectricity). At least a portion of the hydrogen is separated from thefuel side exhaust stream by the PSA hydrogen separation device 29 duringthe fuel cell mode operation. For example, the hydrogen and carbonmonoxide are separated from carbon dioxide and water in the PSA carbondioxide/water separation device or unit 30. Then, at least a portion ofcarbon monoxide is separated from the fuel side exhaust stream (i.e.,from the hydrogen) in the PSA carbon monoxide separation device or unit21. At least a portion of the separated carbon monoxide is recirculatedfrom PSA device 21 through conduits 25 and 81 into the fuel inlet gasstream in the fuel inlet conduit 27 and/or in conduit 53. The separatedhydrogen from the PSA device 29 may also be recirculated into the fuelinlet gas stream through conduits 77 and 81, or provided to a hydrogenstorage vessel or to a hydrogen using device 115 outside the system 1through conduit 83, or both. The valve 79 may be used to determine theportion of the separated hydrogen being provided through conduits 77 and83.

V. Fifth Embodiment

FIGS. 1 and 4 illustrate a system 1 of the fifth embodiment, whichutilizes hydrogen separation from the fuel exhaust stream usingtemperature-assisted pressure swing adsorption. In this system, the fuelcell stack 3 thermal output and the PSA hydrogen separation device 29are thermally integrated. In FIG. 4, the carbon dioxide/water separationunit 30 of the device 29 is illustrated for clarity.

A high temperature fuel cell system 1, such as a SOFC system, produceshydrogen by way of hydrocarbon reforming reactions that occur within thestack 3 and/or within the reformer 9. The hydrogen appears in thesystem's tailgas (i.e., the stack exhaust), and can be effectivelyseparated and purified in a pressure-swing adsorption (PSA) device 29.The gas compression costs associated with the PSA process can beconsiderable. The process of the fifth embodiment takes advantage ofheat available from the other parts of the system 1 to reduce thosecompression costs.

The PSA device 29 operates with a pressure differential between itspressurization/loading and blow-down/purge steps. The pressuredifferential, and the associated change in loading of the adsorbedgases, produces the gas separation. Generally, the higher the pressuredifferential, the more effective the separation and the less amount ofpurge gas required. As shown in the example of FIG. 3, the product gasis typically used as the purge gas, and so its use often needs to beminimized. Typically, the pressure ratio of the pressurization to purgesteps is approximately 10:1, and approximately 10 to 20% of the productgas is lost as purge.

The SOFC stack 3 often operates at pressures near ambient, and thus thefuel exhaust or tailgas containing hydrogen is also near ambient. In atypical non-thermally integrated design, in order to reach pressureseffective for PSA, the tailgas should be compressed by about a factor of10. This pressurization is usually not an issue for hydrogen productionplants that use steam methane reforming, because they often operate atabout 10 atmospheres pressure or above.

If heat is provided during purging steps and removed during loadingsteps, the separation process can be made more effective. This wouldallow a lower level of compression to be used to achieve a similarseparation objective. Alternatively, a higher degree of purification maybe achieved for the same level of compression. This can occur becausethe loading of a adsorbed gas is a strong function of temperature aswell as pressure.

In many separation systems it is not cost-effective to produce andremove this heat. In a high temperature fuel cell system, such as a SOFCsystem, however, heat of an adequate quality is readily available.

In one example shown in FIG. 4, warm exhaust air from the stack 3flowing through oxidizer exhaust conduit 63 is used to heat a PSA columnunder purge 85, and cool inlet air flowing through the oxidizer inletconduit 59 is used to cool a column 83 under load.

In another example, a coolant fluid may circulate between the stack 3operating at an elevated temperature and the PSA separation system 29through a circulation conduit. The fluid removes excess heat from thestack 3 and carries it to the separation system 29, where it is used toheat the gas used to purge the columns. There are a wide variety of waysto achieve these effects using the various heat sources available in ahigh temperature fuel cell system, such as a SOFC system.

Thus, in the system of the fifth embodiment, the compressionrequirements for a pressure-swing adsorption separation system forhydrogen are reduced by using waste heat from a SOFC stack to effect amore efficient separation. Alternatively, a higher degree ofpurification may be achieved for the same level of compression. In otherwords, the extract purity is improved if the column being purged couldbe heated with heat originating from a high temperature fuel cell stack,and the column being fed could be cooled. Alternatively, the pressure ofthe feed gas might be reduced, or the amount of purge gas might bereduced, while the extract purity was maintained. The benefits of thismethod are reduced capital equipment costs associated with compressionand/or higher product value associated with higher purity.

While the method of fifth embodiment was described with respect to thecarbon dioxide/water separation unit 30 of the PSA hydrogen separationdevice 29, it may also be used with the PSA carbon dioxide separationdevice or unit 21 in addition to or instead of the carbon dioxide/waterseparation unit 30. In this case, the waste heat from the hightemperature fuel cell system is used to heat the column under purge inthe carbon monoxide separation device 21 in addition to or instead ofthe column under purge in the carbon dioxide/water separation unit 30.This may be accomplished by providing heat from the oxidizer exhaustconduit 63 to the column under purge in device 21 and/or by providingheat from the coolant fluid in the circulation conduit. If desired, thecolumn under load in the device 21 may be cooled by the cool inlet airflowing through the oxidizer inlet conduit 59.

Since each column of the PSA devices 21 and 29 is alternated betweenbeing the column under load and column under purge, the oxidizer inletconduit 59 and the oxidizer exhaust conduit 63 are thermally integratedwith all columns of each applicable PSA device 21 and/or 29, as shown inFIG. 5. For example, each conduit 59, 63 may be split into as manyparallel branches as there are columns in the respective PSA device.Each branch 59A, 59B, 63A, 63B of the respective conduit 59, 63 isthermally integrated with a respective column 83, 85, of the PSA 29device. The flow of cool or hot air through each branch is controlled bya valve 88. In other words, each PSA device column 83 and 85 isthermally integrated with a branch 59A, 63A and 59B, 63B of bothconduits 59 and 63. However, only cool or warm air is provided to aparticular column depending on if the column is undergoing loading orpurging.

Likewise, if the coolant fluid is circulated between the stack 3operating at an elevated temperature and the PSA separation system 29through a circulation conduit, then the circulation conduit is splitinto parallel branches, and each branch is thermally integrated with arespective PSA column. The flow of the coolant fluid is controlled toeach branch by a valve, such that the warm coolant fluid flows only tothose columns that are undergoing purging.

It should be noted that the term “thermally integrated” in the contextof the fifth embodiment means that the conduit either thermally contactsthe respective PSA column or that it is located adjacent to andpreferably in the same thermal enclosure, such as a hot box or thermalinsulation, as the respective PSA column, to be able to transfer heat tothe column.

FIG. 6 illustrates an alternative example of a PSA unit 21 or 30 of thePSA device 29, where all gas flows are decoupled. In the PSA systemshown in FIG. 6, if valves V1, V2 and V3 (numbered 87, 89 and 91 in FIG.3) are replaced by pairs of two-way valves V1A-V3B (i.e., a two-wayvalve would be placed on each horizontal branch stemming from valvesV1-V3, and locations V1, V2 and V3 corresponding to the valves 87, 89and 91 become “T” shaped conduit junctions), flow restrictors 93 arereplaced by actuating valves V4A and V4B, and the extract line is valved(V5), then all flows can be decoupled. In particular, the blowdown/purgesteps can be decoupled and the pressurization/feed steps can bedecoupled. This may have advantages in terms of improvement of purityand reduction of purge losses, although at the cost of additionalequipment. Furthermore, the lengths of the feed and purge steps can bedecoupled. Either can be of arbitrary length. Clearly this can interruptthe flow of purified extract, and so occasionally additional beds areprovided that increase the system's flexibility and do not interrupt theflow.

VI. Sixth Embodiment

The elements of a system 201 of the sixth embodiment will now bedescribed with respect to FIG. 7. Elements in FIG. 7 with the samenumbers as in FIGS. 1 and 2 should be presumed to be the same unlessnoted otherwise. If desired, the system 201 may be used with any one ormore suitable elements of the first, second, third, fourth and/or fifthembodiments, even if these elements are not explicitly shown in FIG. 7.

In the sixth embodiment, steam methane reformation (SMR) is used topreprocess natural gas before it is fed into the stack 3 forco-generation of hydrogen and electricity from natural gas or otherhydrocarbon fuel using a solid oxide fuel cell system (i.e., aregenerative or a non-regenerative system). SMR transforms methane toreaction products comprising primarily carbon monoxide and hydrogen, asdescribed above. These reaction products are then oxidized in the SOFCstack 3 at high temperature producing electricity. Excess hydrogen isretrieved as a side product. Steam methane reformation reactions areendothermic reactions which require heat, while oxidation reactions inSOFC stack 3 are exothermic reactions which generate heat. This providesa synergy for tight heat integration to improve overall Balance of Plant(BOP) energy efficiency. By integration of reformer 9 and stack 3 in thehot box 108, heat from the stack 3 can be transferred to the reformer 9using convective, radiative and/or conductive heat transfer.

In system 1 illustrated in FIG. 1, the reformer 9 is thermallyintegrated with the stack 3 for heat transfer from the stack 3 to thereformer 9. The stack 3 generates enough heat to conduct the SMRreaction in the reformer 9 during steady-state operation of the system1. However, under some different operating conditions ranging from lowto high stack efficiency and fuel utilization, the exothermic heatgenerated by the stack 3 and provided to the reformer 9 may be ingreater than, the same as or less than the heat required to support thesteam methane reforming reaction in the reformer 9. The heat generatedand/or provided by the stack 3 may be less than required to supportsteam reformation in the reformer 9 due to low fuel utilization, highstack efficiency, heat loss and/or stack failure/turndown. In this case,supplemental heat is supplied to the reformer 9.

In a preferred aspect of the sixth embodiment, the system 201 providesthe supplemental heat to the reformer 9 to carry out the SMR reactionduring steady state operation. The supplemental heat may be providedfrom a burner 15 (more generally referred to in this embodiment as acombustor) which is thermally integrated with the reformer 9 and/or froma cathode (i.e., air) exhaust conduit which is thermally integrated withthe reformer 9. While less preferred, the supplemental heat may also beprovided from the anode (i.e., fuel) exhaust conduit which is thermallyintegrated with the reformer. Preferably, the supplemental heat isprovided from both the combustor 15 which is operating during steadystate operation of the reformer (and not just during start-up) and fromthe cathode (i.e., air) exhaust of the stack 3. Most preferably, thecombustor 15 is in direct contact with the reformer 9, and the stackcathode exhaust conduit 203 is configured such that the cathode exhaustcontacts the reformer 9 and/or wraps around the reformer 9 to facilitateadditional heat transfer. This lowers the combustion heat requirementfor SMR.

Preferably, the reformer 9 is sandwiched between the combustor 15 andone or more stacks 3 to assist heat transfer, as illustrated in FIGS.8-10 and as described in more detail below. The combustor 15, whenattached to the reformer 9, closes the heat balance and providesadditional heat required by the reformer. When no heat is required bythe reformer, the combustor unit acts as a heat exchanger. Thus, thesame combustor (i.e., burner) 15 may be used in both start-up andsteady-state operation of the system 201. When using combustioncatalysts coated on the conduit walls, natural gas is preferablyintroduced at several places in the combustion zone to avoid autoignition and local heating.

Preferably, one or more sensors are located in the system 201 which areused to determine if the reformer requires additional heat and/or howmuch additional heat is required. These sensors may be reformertemperature sensor(s) which measure the reformer temperature and/orprocess parameter sensor(s), which measure one or more of fuelutilization, stack efficiency, heat loss and stack failure/turndown. Theoutput of the sensor(s) is provided to a computer or other processorand/or is displayed to an operator to determine if and/or how muchadditional heat is required by the reformer. The processor or operatorthen controls the combustor heat output based on the step of determiningto provide an desired amount heat from the combustor to the reformer.The combustor heat output may be controlled by controlling the amount offuel and air being provided into the combustor or by shutting off thefuel and/or air being provided into the combustor. The combustor may becontrolled automatically by the processor or manually by operatoractions.

Preferably, the combustor 15 exhaust is provided into the inlet of theair heat exchanger 67 through conduit 205 to heat the air being providedinto the stack 3 through the exchanger 67. Thus, the stack cathodeexhaust is provided to the exchanger 67 indirectly through the combustor15. The configuration of system 201 differs from that of system 1illustrated in FIG. 1 where the stack cathode exhaust is provideddirectly into the exchanger 67 through conduit 63.

The reformer 9 is located in close proximity to the stack 3 to provideradiative and convective heat transfer from the stack 3 to the reformer.Preferably, the cathode exhaust conduit 203 of the stack 3 is in directcontact with the reformer 9 and one or more walls of the reformer 9 maycomprise a wall of the stack cathode exhaust conduit 203. Thus, thecathode exhaust provides convective heat transfer from the stack 3 tothe reformer 9.

Furthermore, if desired, the cathode exhaust from the stack may bewrapped around the reformer 9 by proper ducting and fed to thecombustion zone of the combustor 15 adjacent to the reformer 9 beforeexchanging heat with the incoming air in the external air heat exchanger67, as shown in FIGS. 8-10 and as described in more detail below.Natural gas or other hydrocarbon fuel can be injected and mixed withcathode exhaust air in the combustion zone of the combustor 15 toproduce heat as needed.

FIGS. 8-10 illustrate three exemplary configurations of the stack,reformer and combustor unit in the hot box 108. However, other suitableconfigurations are possible. The reformer 9 and combustor 15 preferablycomprise vessels, such as fluid conduits, that contain suitablecatalysts for SMR reaction and combustion, respectively. The reformer 9and combustor 15 may have gas conduits packed with catalysts and/or thecatalysts may be coated on the walls of the reformer 9 and/or thecombustor 15.

The reformer 9 and combustor 15 unit can be of cylindrical type, asshown in FIG. 8A or plate type as shown in FIGS. 9A and 10A. The platetype unit provides more surface area for heat transfer while thecylindrical type unit is cheaper to manufacture.

Preferably, the reformer 9 and combustor 15 are integrated into the sameenclosure and more preferably share at least one wall, as shown in FIGS.8-10. Preferably, but not necessarily, the reformer 9 and combustor 15are thermally integrated with the stack(s) 3, and may be located in thesame enclosure, but comprise separate vessels from the stack(s) 3 (i.e.,external reformer configuration).

In a preferred configuration of the system 201, fins 209 are provided inthe stack cathode exhaust conduit 203 and in the burner 15 combustionzone 207 to assist with convective heat transfer to the reformer 9. Incase where the reformer 9 shares one or more walls with the cathodeexhaust conduit 203 and/or with the combustion zone 207 of the burner15, then the fins are provided on the external surfaces of the wall(s)of the reformer. In other words, in this case, the reformer 9 isprovided with exterior fins 209 to assist convective heat transfer tothe interior of the reformer 9.

FIGS. 8A and 8B show the cross-sectional top and front views,respectively, of an assembly containing two stacks 3 and a cylindricalreformer 9 combustor 15 unit 210. The combustion zone 207 of thecombustor 15 is located in the core of the cylindrical reformer 9. Inother words, the combustor 15 comprises a catalyst containing channelbounded by the inner wall 211 of the reformer 9. In this configuration,the combustion zone 207 is also the channel for the cathode exhaust gas.The space 215 between the stacks 3 and the outer wall 213 of thereformer 9 comprises the upper portion of the stack cathode exhaustconduit 203. Thus, the reformer inner wall 211 is the outer wall of thecombustor 15 and the reformer outer wall 213 is the inner wall of theupper portion of stack cathode exhaust conduit 203. If desired, acathode exhaust opening 217 can be located in the enclosure 219 toconnect the upper portion 215 of conduit 203 with the lower portions ofthe conduit 203. The enclosure 219 may comprise any suitable containerand preferably comprises a thermally insulating material.

In operation, a natural gas (and/or other hydrocarbon fuel) and steammixture is fed to the lower end of the reformer 9 through conduit 27.The reformed product is provided from the reformer 9 into the stackanode (fuel) inlet 13 through conduit 53. The spent fuel is exhaustedfrom the stack through the anode exhaust 23 and conduit 31.

The air enters the stack through the cathode (air) inlet 19 and exitsthrough exhaust opening 217. The system 201 is preferably configuredsuch that the cathode exhaust (i.e., hot air) exists on the same side ofthe system as the inlet of the reformer 9. For example, as shown in FIG.8B, since the mass flow of hot cathode exhaust is the maximum at thelower end of the device, it supplies the maximum heat where it isneeded, at feed point of the reformer 9 (i.e., the lower portion of thereformer shown in FIG. 8B). In other words, the mass flow of the hot airexiting the stack is maximum adjacent to the lower portion of thereformer 9 where the most heat is needed. However, the cathode exhaustand reformer inlet may be provided in other locations in the system 201.The hot air containing cathode exhaust is preferably but notnecessarily, provided into the combustion zone 207 of the combustor 15through conduit 203.

Natural gas is also injected into the central combustion zone 207 of thecombustor 15 where it mixes with the hot cathode exhaust. The circularor spiral fins are preferably attached to the inner 211 and outer 213reformer walls to assist heat transfer. Heat is transferred to the outerwall 213 of the reformer 9 from the stack 3 by convection and radiation.Heat is transferred to the inner wall 211 of the reformer by convectionand/or conduction from the combustion zone 207. As noted above, thereformer and combustion catalysts can either be coated on the walls orpacked in respective flow channels.

FIGS. 9A and 9B show the cross-sectional top and front views,respectively, of an assembly containing two stacks 3 and a plate typereformer 9 coupled with a plate type combustor 15. The configuration ofthe plate type reformer-combustor unit 220 is the same as thecylindrical reformer-combustor unit 210 shown in FIGS. 8A and 8B, exceptthat the reformer-combustor unit 220 is sandwich shaped between thestacks. In other words, the combustion zone 207 is a channel having arectangular cross sectional shape which is located between two reformer9 portions. The reformer 9 portions comprise channels having arectangular cross sectional shape. The fins 209 are preferably locatedon inner 211 and outer 213 walls of the reformer 9 portions. The platetype reformer and combustion unit 220 provides more surface area forheat transfer compared to the cylindrical unit 210 and also provides alarger cross-sectional area for the exhaust gas to pass through.

FIGS. 10A and 10B show the cross-sectional top and front views,respectively, of an assembly containing one stack 3 and a plate typereformer 9 coupled with a plate type combustor 15. Exhaust gas iswrapped around the reformer 9 from one side. One side of the combustionzone 207 channel faces insulation 219 while the other side faces thereformer 9 inner wall 213.

VII. Seventh Embodiment

Hot box 108 components, such as the stack 3 and reformer 9 are heated toa high temperature before starting (i.e., during the start-up mode) todraw current from the stack as well as produce hydrogen. Furthermore,the stack is preferably run in a reducing environment or ambient usinghydrogen until the stack heats up to a reasonably high temperature belowthe steady state operating temperature to avoid reoxidation of the anodeelectrodes. Stored hydrogen can be used for this process. However, inthe seventh preferred embodiment, a small CPDX (catalytic partialoxidation) unit is used in the start-up mode of the system, to make thesystem independent of external source of hydrogen.

FIG. 7 illustrates the system 201 containing the CPDX unit 223. Anysuitable CPDX device may be used. However, it should be noted that theCPDX unit may be used during start-up of other suitable SOFC systems,such as systems 1 and 101 shown in FIGS. 1 and 2, respectively.

The system 201 preferably also includes a start-up heater 225 forheating the CPDX unit 223 during start-up and a mixer 227 for mixing airand a hydrocarbon fuel, such as methane or methane containing naturalgas. The air and fuel are provided into the mixer through conduits 229and 231, respectively. The mixed air and fuel are provided into the CPDXunit 223 after being mixed in the mixer 227.

The CPDX unit 223 produces hydrogen from the air and fuel mix. Theproduced hydrogen is sent with excess oxygen and nitrogen throughconduit 233 to the reformer 9. The hydrogen passes through the reformer9, the stack 3, the fuel heat exchanger 35, the shift reactor 7 and thecondenser unit 37 and is provided to the combustor 15 through conduit235. The hydrogen is burned in the combustor 15 to heat up the reformer9 and stack 15. This process is continued until the system heats up to acertain temperature, such as a temperature at which oxidation of theanode electrodes is avoided. Then the CPDX unit 223 is stopped or turnedoff, and a hydrocarbon fuel, such as natural gas, is injected directlyinto the combustor 15 through conduit 73 to continue the heatingprocess. The combustor 15 is thermally integrated with the reformer 9and can be used during the start-up and during steady state operationmodes.

VIII. Eighth Embodiment

FIG. 7 illustrates the system 201 with components configured forefficient water management. However, it should be noted that the belowdescribed components may also be configured for efficient watermanagement for other suitable SOFC systems, such as systems 1 and 101shown in FIGS. 1 and 2, respectively.

A SOFC system in general can be self sufficient in water. Heat isrequired to make steam required for the methane reformation. Water fromanode exhaust may be condensed and recycled back to the system.Furthermore, water and natural gas may be fed to a heat exchanger fortransferring heat from the anode exhaust. However, under some operatingconditions, the heat recovered from anode exhaust gas may not besufficient to evaporate all the water needed in the reformation reactionas well as to heat the fuel inlet steam mixture to a desired temperaturebefore feeding this mixture to the reformer. Thus, additional waterheating and management components may be added to the system 201 toevaporate all the water needed in the reformation reaction as well as toheat the fuel inlet steam mixture to a desired temperature beforefeeding this mixture to the reformer.

The system 201 shown in FIG. 7 contains an additional evaporator 237, anoptional supplemental heater/evaporator 239 and a steam/fuel mixer 241.The system operates as follows. The process of steam generation, mixingsteam with fuel, such as natural gas, and preheating mixture may be donein four steps.

First, metered water is provided from the condenser 37 throughcondensate pump 243, water knockout/tank 39, metering pump 47 andoptional water treatment device 245 into the evaporator 237. The meteredwater is heated and at least partially evaporated in the evaporator 237by the heat from the anode exhaust provided into evaporator from theshift reactor 7 through conduit 31.

Second, the partially evaporated water is provided from evaporator 237into the supplemental heater/evaporator 239. Supplemental heat issupplied in the heater/evaporator 239 to complete the evaporationprocess and superheat the steam.

Third, the steam is provided from the heater/evaporator 239 into thesteam/fuel mixer 241. The steam is mixed with the fuel in the mixer.

Fourth, the fuel and steam mix is provided from the mixer 241 into thefuel heat exchanger 35, where the mix is preheated using heat from thehot anode exhaust. The fuel and steam mix is then provided into thereformer through conduit 27.

Water vapor transfer devices such as enthalpy wheels can be added to thesystem to reduce the heat required for the total evaporation process.These devices can transfer water vapor from the anode exhaust toincoming fuel stream.

As described above, the anode exhaust provided into the condenser 37 isseparated into water and hydrogen. The hydrogen is provided from thecondenser 37 via conduit 247 into the conduit 31 leading to the hydrogenpurification subsystem 29 and into conduit 235 leading into thecombustor 15. The flow of hydrogen from condenser 37 through conduits 31and 235 may be controlled by one three way valve or by separate valves249 and 251 located in conduits 31 and 235, respectively. The hydrogenfrom the hydrogen purification system 29 may be provided to theuse/storage subsystem 115 via conduit 83, while the carbon monoxide fromsubsystem 29 is provided to the burner or combustor 15 and carbonmonoxide and water from subsystem 29 are exhausted.

IX. Electricity and Hydrogen Generation

The electrochemical (i.e., high temperature fuel cell) system of thepreferred embodiments of the present invention such as the solid oxideelectrochemical system, such as a SOFC or a SORFC system, or the moltencarbonate fuel cell system, can be used to co-produce hydrogen andelectricity in the fuel cell mode. Thus, while the prior art SORFCsystem can generate either electricity in the fuel cell mode or hydrogenin an electrolysis mode, the system of the preferred embodiments of thepresent invention can co-produce both hydrogen and electricity (i.e.,produce hydrogen and electricity together). The system of the preferredembodiments generates a hydrogen rich exhaust stream using reformingreactions that occur within the fuel cell stack and/or in a reformer inthermal integration with the fuel cell stack. The amount of hydrogenproduced can be controlled by the operator. The hydrogen rich stream isfurther conditioned if necessary and stored or used directly by theoperator. Thus, the high temperature electrochemical systems producepurified hydrogen as a by-product of fuel reformation in the fuel cellmode. The electrochemical system may operate in the fuel cell mode, whenno external electricity input is required, to generate diffusion of ionsacross an electrolyte of the system. In contrast, a reversible orregenerative electrochemical system operates in the electrolysis modewhen external electricity is required to generate diffusion of ionsacross the electrolyte of the system.

It should be noted that the electrochemical system of the preferredembodiments does not necessarily co-produce or co-generate power orelectricity for use outside the system. The system may be operated toprimarily internally reform a carbon and hydrogen containing fuel intohydrogen with minimal power generation or without delivering oroutputting power from the system at all. If desired, a small amount ofpower may be generated and used internally within the system, such as tokeep the system at operating temperature and to power system componentsin addition to other parasitic loads in the system.

Thus, in one aspect of the preferred embodiments of the presentinvention, the high temperature electrochemical system is a SOFC or aSORFC system which co-produces electricity and hydrogen in the fuel cellmode. A SOFC or SORFC system operates in the fuel cell mode when oxygenions diffuse through an electrolyte of the fuel cells from the oxidizerside to the fuel side of the fuel cell containing the carbon andhydrogen containing gas stream. Thus, when the high temperatureelectrochemical system, such as a SOFC or SORFC system operates in thefuel cell mode to generate hydrogen, a separate electrolyzer unitoperating in electrolysis mode and which is operatively connected to thefuel cell stack is not required for generation of hydrogen. Instead, thehydrogen is separated directly from the fuel cell stack fuel sideexhaust gas stream without using additional electricity to operate aseparate electrolyzer unit.

When an SORFC system is used rather than an SOFC system, the SORFCsystem can be connected to a primary source of electricity (e.g., gridpower) and can accept electricity from the primary source when desirableor can deliver electricity to the primary source when desirable. Thus,when operating the SORFC system of the preferred embodiments, the systemoperator does not have to sacrifice electricity production to producehydrogen and vice versa. The SORFC system does not require a hot thermalmass which absorbs heat in the fuel cell mode and which releases heat inthe electrolysis mode for operation or energy storage. However, a hotthermal mass may be used if desired. Furthermore, the system may use,but does not require a fuel reformer.

Furthermore, a relative amount of hydrogen and electricity produced canbe freely controlled. All or a portion of the hydrogen in the fuel sideexhaust stream may be recirculated into the fuel inlet stream to providecontrol of the amount of electricity and hydrogen being co-produced inthe system, as will be described in more detail below. The hydrogenproduct can be further conditioned, if necessary, and stored or useddirectly in a variety of applications, such as transportation, powergeneration, cooling, hydrogenation reactions, or semiconductormanufacture, either in a pressurized or a near ambient state.

The system 1 or 101 shown in FIGS. 1 and 2 derives power from theoxidation of a carbon and hydrogen containing fuel, such as ahydrocarbon fuel, such as methane, natural gas which contains methanewith hydrogen and other gases, propane or other biogas, or a mixture ofa carbon fuel, such as carbon monoxide, oxygenated carbon containinggas, such as methanol, or other carbon containing gas with a hydrogencontaining gas, such as water vapor, H₂ gas or their mixtures. Forexample, the mixture may comprise syngas derived from coal or naturalgas reformation. Free hydrogen is carried in several of the systemprocess flow streams. The carbon containing fuel is provided into thesystem from a fuel source, which may comprise a fuel inlet into the fuelcell stack, a fuel supply conduit and/or a fuel storage vessel.

The fuel cell stack 3 preferably contains the fuel cells, separatorplates, seals, gas conduits, heaters, thermal insulation, controlelectronics and various other suitable elements used in fuel cellstacks.

The system 1, 101 and 201 also contains at least one hydrogen separator,such as the PSA hydrogen separation device 29. The system 1, 101 and 201also contains an optional hydrogen conditioner 114, as shown in FIGS. 1and 2. The hydrogen conditioner 114 may be any suitable device which canpurify, dry, compress (i.e., a compressor), or otherwise change thestate point of the hydrogen-rich gas stream provided from the hydrogenseparator 29. If desired, the hydrogen conditioner 114 may be omitted.

The system 1, 101 and 201 also contains a hydrogen storage/use subsystem115, as shown in FIG. 2. This subsystem 115 may comprise a hydrogenstorage vessel, such as a hydrogen storage tank, a hydrogen dispenser,such as a conduit which provides hydrogen or a hydrogen-rich stream to adevice which uses hydrogen, or a hydrogen using device. For example, thesubsystem 115 may comprise a conduit leading to a hydrogen using deviceor the hydrogen using device itself, used in transportation, powergeneration, cooling, hydrogenation reactions, or semiconductormanufacture.

For example, the system 1, 101 and 201 may be located in a chemical or asemiconductor plant to provide primary or secondary (i.e., backup) powerfor the plant as well as hydrogen for use in hydrogenation (i.e.,passivation of semiconductor device) or other chemical reactions whichrequire hydrogen that are carried out in the plant.

Alternatively, the subsystem 115 may also comprise another fuel cell,such as an SOFC or SORFC or any other fuel cell, which uses hydrogen asa fuel. Thus, the hydrogen from the system 1, 101 and 201 is provided asfuel to one or more additional fuel cells 115. For example, the system1, 101 and 201 may be located in a stationary location, such as abuilding or an area outside or below a building and is used to providepower to the building. The additional fuel cells 115 may be located invehicles located in a garage or a parking area adjacent to thestationary location. In this case, the carbon and hydrogen containingfuel is provided to the system 1, 101 and 201 to generate electricityfor the building and to generate hydrogen which is provided as fuel tothe fuel cell 115 powered vehicles. The generated hydrogen may be storedtemporarily in a storage vessel and then provided from the storagevessel to the vehicle fuel cells 115 on demand (analogous to a gasstation) or the generated hydrogen may be provided directly from thesystem 1, 101 and 201 to the vehicle fuel cells 115.

In one preferred aspect of the present invention, the hydrogen separator29 is used to separate and route hydrogen from the fuel side exhauststream only into the subsystem 115. In another preferred aspect of thepresent invention, the hydrogen separator 29 is used to separatehydrogen from the fuel side exhaust stream and to route all or a part ofthe hydrogen back into the fuel inlet 13 of the fuel cell stack 3through conduit 81, to route all or part of the hydrogen to thesubsystem 115 and/or to route the hydrogen out with the tail gas.

A preferred method of operating the systems 1, 101 and 201 will now bedescribed. The systems are preferably operated so that excess fuel isprovided to the fuel cell stack 3. Any suitable carbon containing andhydrogen containing fuel is provided into the fuel cell stack. The fuelmay comprise a fuel such as a hydrocarbon fuel, such as methane, naturalgas which contains methane with hydrogen and other gases, propane orother biogas. Preferably, an unreformed hydrocarbon fuel from theby-pass valve 11 and a hydrogen fuel from the reformer 9 are providedinto the stack 3.

Alternatively, the fuel may comprise a mixture of a non-hydrocarboncarbon containing gas, such as carbon monoxide, carbon dioxide,oxygenated carbon containing gas such as methanol or other carboncontaining gas with a hydrogen containing gas, such a water vapor orhydrogen gas, for example the mixture may comprise syngas derived fromcoal or natural gas reformation. The hydrogen and water vapor may berecycled from the fuel side exhaust gas stream or provided from hydrogenand water vapor conduits or storage vessels.

The reformation reactions occur within the fuel cell stack 3 and/or inthe reformer 9 and result in the formation of free hydrogen in the fuelside exhaust gas stream. For example, if a hydrocarbon gas such asmethane is used as a fuel, then the methane is reformed to form amixture containing non-utilized hydrogen, carbon dioxide and water vaporin the fuel cell stack 3. If natural gas is used as a fuel, then thenatural gas may be converted to methane in a preprocessing subsystem orit may be reformed directly to a non-hydrocarbon carbon containing gassuch as carbon monoxide in the reformer 9.

Preferably, the fraction of hydrogen separated by the hydrogen separator29 and the amount of total fuel provided to the fuel cell stack 3 forelectricity and hydrogen production are variable and under the controlof an operator operating a control unit of the system. An operator maybe a human operator who controls the hydrogen separation and electricityproduction or a computer which automatically adjusts the amount ofhydrogen separation and electricity production based on predeterminedcriteria, such as time, and/or based on received outside data orrequest, such as a demand for electricity by the power grid and/or ademand for hydrogen by the subsystem 115. Controlling these twoparameters allows the operator to specify largely independently theamount of hydrogen produced and the amount of electricity generated. Theoutside data or request may comprise one or more of electricity demand,hydrogen demand, electricity price and hydrogen price, which may betransmitted electronically to a computer system operator or visually oraudibly to a human system operator.

In one extreme, when the user of the system needs electricity, but doesnot need additional hydrogen, then the operator can choose to have thehydrogen containing streams recirculated back into the fuel cell stack 3by the separator 29 through conduit 81 by opening valve 79, whileproviding no hydrogen or a minimum amount of hydrogen to the subsystem115, through conduit 83, where hydrogen flow may also be controlled by avalve.

In another extreme, when the user of the system needs hydrogen, but doesnot need any electricity generated, the operator can choose to have thefuel cell stack 3 act primarily to internally reform the carboncontaining fuel into hydrogen with minimal power generation and/orminimal or no external power output/delivery from the system. A smallamount of power may be generated to keep the system at operatingtemperature and to power the hydrogen separator 29 and conditioner 114,if necessary, in addition to other parasitic loads in the system. All ormost of the hydrogen from the separator 29 is provided to the subsystem115 rather than to the conduit 81. In this case, additional water fromthe water supply 39 is preferably added to the fuel.

In the continuum between the two extremes, varying amounts of hydrogenand electricity may be needed simultaneously. In this case, the operatorcan choose to divert varying amounts of the hydrogen from the separator29 to conduits 81 and 83, while simultaneously generating the desiredamount of electricity. For example, if more hydrogen is recirculatedback into the fuel cell stack 3 through conduit 81 by controlling valve79, then more electricity is generated but less hydrogen is availablefor use or storage in the subsystem 115. The trade off between theamount of electricity and hydrogen produced can vary based on the demandand the price of each.

The trade off between the amount of electricity and hydrogen generatedmay also be achieved using several other methods. In one method, theamount of fuel provided to the fuel cell stack 3 is kept constant, butthe amount of current drawn from the stack 3 is varied. If the amount ofcurrent drawn is decreased, then the amount of hydrogen provided to thehydrogen separator 29 is increased, and vice versa. When less current isdrawn, less oxygen diffuses through the electrolyte of the fuel cell.Since the reactions which produce free hydrogen (i.e., the steam-methanereforming reaction (if methane is used as a fuel) and the water-gasshift reaction) are substantially independent of the electrochemicalreaction, the decreased amount of diffused oxygen generally does notsubstantially decrease the amount of free hydrogen provided in the fuelside exhaust gas stream.

In an alternative method, the amount of current drawn from the stack iskept constant, but the amount of fuel provided to the stack 3 is varied.If the amount of fuel provided to the stack 3 is increased, then theamount of hydrogen provided to the hydrogen separator 29 is increased,and vice versa. The amount of fuel may be varied by controlling the flowof fuel through the fuel inlet conduit 27 by a computer or operatorcontrolled valve 28 and/or by controlling the flow of fuel through theby-pass line 11 by valve 55.

In another alternative method, both the amount of current drawn and theamount of fuel provided into the fuel cell stack 3 are varied. Theamount of hydrogen generated generally increases with decreasing amountsof drawn current and with increasing amounts of fuel provided into thefuel cell stack. The amount of hydrogen generated generally decreaseswith increasing amounts of drawn current and with decreasing amounts offuel provided into the fuel cell stack.

Preferably, the systems of the preferred embodiments may be operated atany suitable fuel utilization rate. Thus, 0 to 100 percent of the fuelmay be utilized for electricity production. Preferably, 50 to 80 percentof the fuel is utilized for electricity production and at least 10percent, such as 20 to 50 percent, of the fuel is utilized for hydrogenproduction. For example, a 100 kWe SOFC system may be used to generatefrom about 70 to about 110 kWe of electricity and from about 45 to about110 kg/day of high pressure hydrogen when 50 to 80 percent of the fuelis utilized for electricity production. The systems of the preferredembodiments may be used to produce hydrogen cost effectively. Thus, themethod of the preferred embodiments provides a reduction in the cost ofhydrogen production.

If the fuel cell stack 3 is a solid oxide regenerative fuel cell (SORFC)stack which is connected to a primary source of power (such as a powergrid) and a source of oxidized fuel (such as water, with or withoutcarbon dioxide), then the device can operate transiently in anelectrolysis mode as an electrolyzer to generate hydrogen streams,methane streams, or mixtures when economically advantageous (e.g., whenthe cost of electricity is inexpensive compared to the cost of the fuelcontaining bound hydrogen), or during times when the demand for hydrogensignificantly exceeds the demand for electricity. At other times, thesystem 1, 101 and 201 can be used in the fuel cell mode to generateelectricity from the stored hydrogen or carbon containing fuel. Thus,the system 1, 101 and 201 can be used for peak shaving.

The fuel cell systems described herein may have other embodiments andconfigurations, as desired. Other components, such as fuel side exhauststream condensers, heat exchangers, heat-driven heat pumps, turbines,additional gas separation devices, hydrogen separators which separatehydrogen from the fuel exhaust and provide hydrogen for external use,fuel preprocessing subsystems, fuel reformers and/or water-gas shiftreactors, may be added if desired, as described, for example, in U.S.application Ser. No. 10/300,021, filed on Nov. 20, 2002, in U.S.Provisional Application Ser. No. 60/461,190, filed on Apr. 9, 2003, andin U.S. application Ser. No. 10/446,704, filed on May 29, 2003 allincorporated herein by reference in their entirety. Furthermore, itshould be understood that any system element or method step described inany embodiment and/or illustrated in any figure herein may also be usedin systems and/or methods of other suitable embodiments described above,even if such use is not expressly described.

The foregoing description of the invention has been presented forpurposes of illustration and description. It is not intended to beexhaustive or to limit the invention to the precise form disclosed, andmodifications and variations are possible in light of the aboveteachings or may be acquired from practice of the invention. Thedescription was chosen in order to explain the principles of theinvention and its practical application. It is intended that the scopeof the invention be defined by the claims appended hereto, and theirequivalents.

1. A solid oxide fuel cell system, comprising: a solid oxide fuel cellstack; a first means for reforming a hydrocarbon fuel to a hydrogencontaining reaction product and for providing the reaction product tothe stack; and a second means for heating the first means using at leastone of a stack cathode exhaust, a heat from a combustion reaction, andat least one of radiative and convective heating from the stack.
 2. Thesystem of claim 1, wherein the second means for heating the first meansuses at least two of the stack cathode exhaust, the heat from acombustion reaction, and the at least one of radiative and convectiveheating from the stack.
 3. The system of claim 1, wherein the secondmeans for heating the first means uses the stack cathode exhaust, theheat from a combustion reaction, and the at least one of radiative andconvective heating from the stack.
 4. The system of claim 1, furthercomprising a third means for receiving the stack cathode exhaust and ahydrocarbon fuel and for heating the first means using a combustionreaction of the stack cathode exhaust and the hydrocarbon fuel duringsteady state operation of the stack.